Packet Switched Networks are replacing legacy Time Division Multiplexing (TDM) based networks, such as Synchronous Digital Hierarchy (SDH) networks, for their capability to handle data traffic like Ethernet and IP in a more optimised and flexible manner. Connection Oriented Packet Switched (CO-PS) Networks are an ideal candidate for the migration from legacy TDM networks toward all-packet networks for their capability of end-to-end resilience and performance monitoring and for their manageability.
In order to replace legacy SDH networks, CO-PS Networks are required to guarantee transport-grade performance and provide the same resilience to faults offered by legacy transport networks. Typically, transport networks are required to switch to a protection path within 50 ms of a fault occurring. The sub 50 ms protection switching requirement is a difficult requirement to fulfill in SDH networks, especially when a high number of simultaneous protection switching instances needed to be managed, and it is an even greater challenge for CO-PS Networks.
One type of CO-PS network technology is Multi-Protocol Label Switching Transport Profile (MPLS-TP), which is being developed as a transport profile of Multi-Protocol Label Switching (MPLS). MPLS-TP aims to provide a similar “look and feel” as SDH with inherent interoperability capability with existing IP/MPLS networks. In MPLS-TP networks Label Switched Path (LSP) tunnels are created between network nodes which are required to communicate with one another. Transport units (e.g. packets) include a shim header which contains the label that identifies a specific tunnel. In order to increase the availability of the network, LSP tunnels usually need to be made redundant, using some network protection scheme. In the case of MPLS-TP this typically means 1+1 or 1:1 end-to-end protection schemes.
In MPLS, which can also be used as a CO technology, no end-to-end protection exists but only local repair mechanisms and restoration. A problem is that when a high number of LSP tunnels are present in a network and need to be protected, there is likely to be a high number of simultaneous protection switches at a node. The number of simultaneous protection switch events that may take place at a node are dependent upon traffic relationships between the nodes, network topology and network design, as well as the type of fault that has occurred. Protection is governed by Operations, Administration and Management (OAM) packets which, for fast protection, are sent at short intervals (typically 3.3 ms) and need to be processed by the receiving node. Mechanisms for declaring a failure of an LSP tunnel include: lack of connectivity verification (e.g. three consecutive periodic OAM packets are not received) or explicit fault indication messages such as Forward Defect Indication (FDI).
The number of protection instances that may need to be simultaneously switched at a node can be high. In addition, where traffic is carried over LSP tunnels by Pseudo Wires (PW), which is the typical case for MPLS-TP to transport Ethernet and TDM/ATM clients, the PW level can be protected with another level of OAM, which further increases the number of protection switches that must be performed. One possible solution to meet the 50 ms protection switching requirement is LSP nesting. LSP nesting creates a hierarchy of LSPs so that a multitude of inner LSPs are transported by an outer LSP for a given network portion. The outer LSP is the entity on which traffic protection is performed in that network portion. This requires the creation of an additional layer in the network and has the restriction of providing a solution only in that network portion.
International Telecommunications Union (ITU-T) Recommendation G.808.1 “Generic Protection Switching—Linear trail subnetwork protection” describes a mechanism called Group Trail Protection. A group of working path and protection path connections are configured between a common pair of network end points. All working paths and protection paths are required to connect the same pair of end nodes and to follow the same route. Logic at the receiving node merges individual trail signal fail (TSF) signals into a single SF Group (SFG) and merges individual trail signal degrade (TSD) signals into a single SD Group (SDG). In the case of 1:1 protection, an Automatic Protection Switching (APS) message has to be sent for the whole group. Protection is activated for the entire group when the SFG signal is active. Three policies are described by G.808.1 to generate an SFG: (1) all members of the protection group are failed, i.e. SFG is declared in case individual signals are in TSF; (2) one selected member is failed, i.e. an individual signal is chosen as reference signal and SFG is declared in case TSF of the reference signal is active; (3) a given percentage of members are failed, i.e. only if the number of active TSF exceeds a given threshold, SFG is active.
The present invention seeks to provide an alternative method of protecting traffic in a network.